This study is an exercise intervention trial for young women. It is based on the hypotheses that 1) daily activities can be expressed as a series of discrete events, each represented by frequency and intensity terms, 2) that density of any bone reflects its daily loading history, and 3) that intensity of activity is more important to bone density than the number of loading cycles. Women will be randomized to 4 groups: control, high intensity exercise (weight lifting), low intensity-high repetition exercise (jogging), and combined jogging and weight lifting, each program continuing for 5 months. Total bone mineral and regional bone densities in 5 anatomic regions (calcaneus, knee, hip, low back, shoulder) will be measured on entry by dual photon absorptiometry, and will be repeated on completion of the study. Average loading histories for each site will be computed from log books and exercise records by methods validated previously in our laboratory. Initial values and changes in total and regional bone mass will be compared among groups, and correlated with average loading histories. Data analysis will permit correlation of initial bone mass values with pre-exercise loading history, comparison of changes in bone mass among groups, correlation of changes in bone densities with computed loading history at each anatomic site, and independent determination of the stress exponent, "m".